An alternate road deicer is needed to replace the nine million tons of salt (NaCl) used annually in the U.S. Although both inexpensive and effective, salt causes enormous economic losses annually from corrosion of vehicles, bridges and underground utilities, from deterioration of concrete roads and bridges, from pollution of streams and water supplies, and from killing roadside vegetation. Pollution of water supplies with NaCl is of particular concern in states such as in Massachusetts, where the sodium content in drinking water in many communities already exceeds 20 mg/L, the recommended upper limit for individuals on a sodium-restricted diet.
As a result of research sponsored by the Federal Highway Administration, calcium magnesium acetate (CMA) has been identified as almost equivalent to salt (NaCl) in deicing properties without the harmful side effects. It is nontoxic, noncorrosive, nonpolluting and has a lower freezing point depression than salt. The freezing point depression is -23.degree. C. for CMA with an equimolar ratio and decreases for mixtures that contain higher ratios of magnesium until the optimum (eutectic) composition is reached at a Mg:Ca mole ratio of approximately 2.3 and a freezing point of -37.degree. C., compared with -21.degree. C. for NaCl. Widespread use of CMA would also help to alleviate the effects of acid rain by acting as a buffer and helping to neutralize sulfuric and nitric acids in the environment adjacent to roadways and in streams and lakes that receive the runoff. CMA has performed satisfactorily in field evaluations, except that a somewhat higher application rate than for NaCl was required for equivalent deicing.
The major deterrent to CMA use is cost. Approximately 80% of CMA manufacturing cost is for acetic acid, which lists for $0.29/lb. plus shipping. For example "ICE-B-GON manufactured by Chevron Chemical Co. from glacial acetic acid, magnesia (MgO) and dolomitic lime products such as dolime (CaO.MgO) or hydrated dolime [Ca(OH).sub.2 . Mg (OH).sub.2 ], currently costs $0.30/lb. f.o.b. plant for the 91% purity product.
Although the short term cost of deicing with CMA, including the costs of equipment depreciation and labor, may be five to ten-fold higher than for NaCl due to manufacturing costs, the long term savings from CMA use would be much higher than the CMA cost because of reduced damage to water supplies, vehicles, underground utilities, road side vegetation and the nation's highway infrastructure. As a result, the economic and environmental advantages of using CMA would be more effectively realized by lowering the manufacturing costs of CMA.
Two methods have been proposed for reacting acetic acid with calcium and magnesium oxides to prepare CMA. Method I reacts dolomitic lime products [CaO.MgO or Ca(OH).sub.2. Mg(OH).sub.2 ] and MgO with glacial acetic acid which has been diluted with approximately 10-20% water, needed to help remove the heat released during neutralization and prevent an excessive temperature rise, which might create a safety hazard from the possibility of explosive combustion of acetic acid vapor with air. The CMA product is formed into spherical pellets, as for "ICE-B-GON," and dried to remove most of the residual water. Projected cost, estimated in 1986 for production of 40,000 tons/yr, was $0.215/lb. CMA, based on acetic acid at $0.250/lb. and dolime (CaO.MgO) at $0.045/lb. ($90/ton). The CMA for that study was made with an equimolar Ca:Mg ratio. To produce CMA with a 3Ca:7Mg ratio requires substitution for part of the dolime with magnesia, thereby adding to the projected CMA production cost, size MgO lists for $0.20/lb. plus shipping.
Method II is another plausible and potentially cost-competitive approach to CMA production that has not been commercialized in which dilute acetic acid made by fermentation is neutralized with dolomitic lime products and magnesia, followed by bacterial cell separation, liquid concentration to remove most of the water in multiple-effect evaporators or vapor recompression evaporators, and drying the final product. In this approach, the acetic acid would be made from renewable resources (grains, wood or crop residues) or even from carbon monoxide, carbon dioxide or hydrogen gases. An anaerobic thermophilic microorganism, Clostridium thermoaceticum was proposed because the theoretical conversion cf glucose to acetic acid was 100%, with 85% found experimentally. Other thermophilic acetogenic bacteria such as Clostridium thermoautotroohicum and Acetocenium kivui may also be used. However, these microorganisms cannot tolerate a low pH and their growth rate and rate of production of acetic acid decline drastically as the concentration of free acetic acid increases.
The prior art teaches that pH-controlled C. thermoaceticum fermentations with NaOH additions to form sodium acetate increased acetate concentration threefold compared with fermentations without pH control. Commercial success, therefore, would depend on neutralizing the acetic acid to form CMA as soon as it is formed and on cultures that are tolerant to CMA solutions, thus allowing a higher concentration of CMA to be attained in the fermentation broth, which would reduce the amount of water that must be evaporated during the liquid concentration step prior to drying.
Method II can also be applied to fermentation broths of Acetobacter strains to form CMA during the fermentation or in the clarified vinegar resulting from that type fermentation. Acetobacter strains used to make vinegar are more tolerant to acetic acid, but conversion yields are lower, 67% theoretical with 60-65% realized in production. The ret cost to produce CMA by Method II from corn at $2.00/bushel and using either Acetobacter or Clostridium thermoaceticum fermentation broths was estimated to be approximately the same, $0.17/lb., because the by-product credits from the vinegar process were more than those from the higher-yielding thermophilic fermentation and more water had to be evaporated in the latter process.
Therefore there is a need for a neutralization process which reduces the costs of producing CMA by reducing the need for MgO in preparing a 2.3 Mg/Ca product.
Also, there exists a need for a less expensive process for effectively manufacturing CMA from fermentation broths while controlling pH within a range for microorganisms to actively grow and produce acetic acid.
There is also a need for a process to reduce the cost of CMA production by use of decentralized processing plants that use locally available raw materials such as corn to produce a concentrated liquid CMA product to serve local areas, thus avoiding the costs of drying, handling and storing a granular solid.
Additionally, a need exists for a process to reduce the cost of CMA production by means of cocurrent production of valuable by-products in addition to corn germ, fiber and animal feed recovered from the milling and fermentation processes which may be sold to help offset the costs of CMA production.
Finally, there is a need for a method to react MgO.CaCO.sub.3 with acetic acid solutions in a vertical cascade reaction equipped with rotating mixing elements and separate the CO.sub.2 in each stage, thus avoiding the interference from CO.sub.2 bubbles rising between stages causing a large amount of CO.sub.2 to flood the top of the reactor.